You calculate the length of pregnancy from the date of the first day of your last menstrual period so even though it was approx 2 weeks since you'd ovulated, you'd have actually been approx 4 weeks pregnant.
It's likely that you had a chemical pregnancy, which is when the egg is fertilised but does not implant properly and so you come on to a period. In the old days, women would have assumed they had just had a normal period, or perhaps a slightly late one but nowadays the accurate early detection tests mean that very early pregnancy can be detected.
I have had a suspected chemical pregnancy myself and I did not need to go and see my doctor but if you continue to bleed heavily or get very bad cramps then it would be a good idea to get things checked out.
